IN my letter of June 19 I briefly outlined three arguments which consistently point to a concentration of the radio-active elements in the earth's crust, such concentration having been accomplished at the expense of the material of the interior. The first of these arguments (in which the limited distribution of the radio-elements is deduced from the earth's temperature-gradient) is made the basis of a revival of Arrhenius's view that radio-active disintegration may be inhibited under the conditions prevailing at great depths (NATURE, June 26, July 10, and July 17). Thus, in place of the deduction that the amounts of uranium and thorium existing in the earth's interior are negligible, even thermally negligible, is put forward the alternative deduction that these parent elements are not necessarily absent, but only temporarily impotent, their output of energy, by which alone we could be aware of them, being inhibited by the enormous pressures to which they are subjected.